1. Field of the Invention
The invention relates to the use of water-insoluble poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers for defoaming aqueous media.
In numerous industrial processes, aqueous formulations use surface-active substances in order, for example, to emulsify water-insoluble substances or to reduce the interfacial tension and so achieve better wetting characteristics. An unwanted accompanying effect of these surface-active substances, however, is that air introduced during the preparation and processing of such water-based formulations is stabilized as foam. Examples of processes in which such problems occur include papermaking, wastewater treatment, emulsion polymerization, and the manufacture and application of water-dilutable coating systems.
Foam of this kind, produced in the course of stirring or dispersing operations during manufacture or filling, prolongs production times and reduces the effective capacity of the production plant. Foam is disruptive to the application of coating materials since it leads to unwanted surface defects. As a result, the use of antifoams or defoamers in virtually all water-based systems is widespread and often indispensable.
In the past, a large number of formulations have been described which envisage the use of, for example, silicone oils, organomodified siloxanes, hydrophobic polyoxyalkylenes, mineral oils, natural oils and other hydrophobic liquids as defoaming substances. Frequently, the above-mentioned substances are used in combinations with hydrophobic solids, such as pyrogenic silicas, metal stearates or fatty acid amides, for example, which often intensify the foam inhibiting or defoaming effect.
In accordance with the present state of knowledge, it is critical to the defoaming action of a defoamer that it is able to penetrate into the foam lamellae and to weaken them until they collapse. To achieve this requires a controlled incompatibility (hydrophobicity) with the aqueous phase to be defoamed: if a defoamer is too compatible (hydrophilic), it will not be able to be very effective since it does not penetrate preferentially into the foam lamella. If the degree of incompatibility is too great, then defoaming is generally very good but there are frequently the unwanted side effects of surface defects, deterioration in wetting characteristics, and separation phenomena.
Consequently, the search for a suitable defoamer is always a search for the correct incompatibility/compatibility balance for the system to be defoamed, with the aim of getting as close as possible to the target hydrophobicity/hydrophilicity equilibrium.
2. Description of the Related Art
In the past, polyoxyalkylene-polysiloxane block copolymers have proven suitable for allowing this equilibrium to be established in a particularly variable manner, as is described, inter alia, in U.S. Pat No. 3,763,021.
When polyoxyalkylene-polysiloxane block copolymers are used, advantage is taken of the fact that the polysiloxane blocks can be modified in a targeted manner with polyoxyalkylene units which, by virtue of their composition from hydrophilic and hydrophobic polyoxyalkylene units, can be adapted, in interplay with the polysiloxane block, to the abovementioned desired hydrophobicity/hydrophilicity equilibrium.
The prior art, an example of which that may be mentioned here is DE-C-1 012 602, includes polyoxyalkylene-polysiloxane block copolymers in which the polyoxyalkylene block is defined as (CnH2nO)x. The index n is an integer from 2 to 4, it being essential that these polyoxyalkylene blocks always include xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 groups as a common structural element, R being a hydrogen atom, a methyl group or an ethyl group. Such polyoxyalkylene blocks are prepared by ionic addition polymerization of oxirane derivatives, such as ethylene oxide, propylene oxide and butylene oxide. The basic structure of the properties-determining polyoxyalkylene chain is always, however, the continually repeating sequence of the two carbon atoms and one oxygen atom along the polyoxy-alkylene chain. Nor is this principle altered by the use of epoxides of longer-chain olefins.
Typically, however, polyoxyethylenes and polyoxy-propylenes are used in the polyoxyalkylene-polysiloxane block copolymers of the prior art.
By varying the oxyalkylene units, and in particular by modifying their respective proportions and their sequence in the polymer (random and/or in blocks), a large number of polyoxyalkylene-polysiloxane block copolymers are obtainable.
Because of the restriction to these basic units described above, however, in many cases it has not been possible to date to find the optimum incompatibility/compatibility balance required for certain defoamers.
Therefore, an object of the present invention to provide particularly suitable polyoxyalkylene-polysiloxane block copolymers for defoaming aqueous media, said copolymers making it possible to establish the above-described desired incompatibility/compatibility balance in a targeted manner better than as has been possible to date.
Surprisingly, this object is achieved by the use of water-insoluble poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers of the general average formula I: 
where
R1 and/or R7 are R2 or xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8,
R2 and R3 are identical or different alkyl radicals, preferably having 1 to 24 carbon atoms, or are unsubstituted or substituted aryl radicals, preferably having up to 24 carbon atoms, with phenyl being especially preferred.
R4 is a divalent radical of the formula xe2x80x94Oxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94NR2xe2x80x94, xe2x80x94Sxe2x80x94 or of the formula
xe2x80x83xe2x80x94[Oxe2x80x94Si(CH3)2]uxe2x80x94
where
xe2x80x83u is from 2 to 200,
R5 is identical or different alkylene radicals, preferably having-1 to 24 carbon atoms, or is
xe2x80x94CnH2n-fR2lxe2x80x94Oxe2x80x94CmH2m-gR2gxe2x80x94
xe2x80x83where
f is from 0 to 12,
g is from 0 to 12,
n is from 1 to 18,
m is from 1 to 18,
R6 is identical or different radicals of the general formula
xe2x80x94Oxe2x80x94(C2H4-aR9aO)b(CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2-cR10cO)dxe2x80x94
xe2x80x83where
R9 is identical or different alkyl radicals, preferably having 1 to 4 carbon atoms.
R10 is an alkyl radical, preferably having 1 to 6 carbon atoms,
but where, taking into account all radicals R6, the proportion of C2H4O groups
among the total of all radicals R6 does not exceed 40% by weight,
a is from 0 to 3,
b is from 0 to 200,
c is from 0 to 2,
d is from 2 to 40,
the sum (b+d)=3 to 240
and the sequence of the individual polyoxyalkylene segments xe2x80x94(C2H4-aR9aO)bxe2x80x94 and xe2x80x94(CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2-cR10cO)dxe2x80x94 can be as desired and, in particular, embraces block copolymers, such as random polymers and combinations thereof,
R8 is a hydrogen, a substituted or unsubstituted alkyl radical, preferably having from 1 to 6 carbon atoms, or an acyl radical,
v is from 3 to 200,
w is from 0 or 1,
x is from 0 or 1,
y is from 0 to 200,
and, if y=0, R1 and/or
R7 are/is xe2x80x94[R4]wxe2x80x94[R5]xxe2x80x94[R6]xe2x80x94R8.
In many cases, polyoxyalkylene-polysiloxane block copolymers, which have proven highly suitable, are those obtained by means of combinations of two polyoxyalkylene units with one or more polysiloxane copolymers. In that case, the distinction in the case of the polyoxyalkylene copolymers can be a distinction in relation either to the relative proportion of the units used for the preparation of this copolymer or to the molecular weight of the whole polyoxyalkylene unit, or else can lie in its functionality.
Different polysiloxane copolymers can be distinguished in accordance with both their molecular mass and their degree of branching or the number and/or relative position of the reactive groups used which are available for linkage.
The mode of action of the compounds of the invention is expressly not that they bring about particularly good compatibilization between constituents of a mixture that differ in polarity. Rather, it is the case that they themselves are just compatible enough not to induce any unwanted side effects such as surface defects, deteriorations in the wetting characteristics, or separation phenomena, without bringing about the compatibilization of other constituents.
A decisive factor in comparison with the prior art is that the compounds of the invention deviate from the above-described xe2x80x9cstandard construction principlexe2x80x9d of the xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 groups and instead contain xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94 groups, which also have a particularly positive effect on the incompatibility/compatibility balance which is essential for effective defoaming without unwanted side effects.
Such poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers can be prepared in accordance with customary methods as in the prior art. As the skilled worker is well aware, such methods include, in particular, the hydrosilylation reaction of additionally olefinically unsaturated poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers with hydropolysiloxanes or the nucleophilic substitution of poly(oxy-1,4-butanedyiy)-containing polyoxyalkylene-polysiloxane block copolymers with nucleophilic groups on polysiloxanes containing at least one silicon atom substituted by an electronegative group. Processes of this kind and the preparation of appropriate polysiloxanes are described, for example, in W. Noll xe2x80x9cChemie undechnologie der Siliconexe2x80x9d, Verlag Chemie, Weinheim, 1968.
In accordance with the prior art, the poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers required for this purpose can be obtained by ionic polymerization of tetrahydrofuran and its derivatives and of appropriately substituted oxiran derivatives.
An important feature of the present invention that the poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers of the invention are not water-soluble at the customary processing temperature of the media to be defoamed. This means that a 1:1 mixture (% by weight) with water leads to a nonhomogeneous mixture of the two phases and that no dissolving of the poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers of the invention takes place. However, small fractions of free polyoxyalkylenes may be present in the poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers as a result of their preparation, as set out below. Since the preparation of these copolymers essentially obeys the laws of statistics, it is also possible for particularly oxyethylene-rich polyoxyalkylenes to occur, which in turn may be soluble in water. These fractions, however are very small and in the poly(oxy-1,4-butanediyl)-containing polyoxyalkylene-polysiloxane block copolymers of the invention are always less than 10% by weight.